1. Field of the Invention
The invention relates to emitter coupled output circuits. More particularly, the invention concerns an integrated circuit having a pair of emitter coupled output transistors and an electrical connection area that is connectable to an external circuit to program the magnitude of the voltage swings of the outputs.
2. Description of the Related Art
Output buffers are common components of high speed logic circuits. FIG. 1 illustrates a typical prior art current mode logic (CML) output buffer circuit 100 that has emitter coupled outputs. In the buffer circuit 100, the emitters of output transistors Ql and Q2 are connected together, and are thus "emitter coupled." Resistor R1 is connected to the collector of transistor Q1, and resistor R2 is connected to the collector of transistor Q2. The collector of output transistor Q2 is connected to output Y, and the collector of output transistor Q1 is connected to complementary output YN. The output current 1101 for these emitter coupled outputs is set by a constant current source consisting of transistor Q3 and resistor R3. The amount of current through the constant current source determines the magnitudes of the voltage swings of the outputs Y and YN.
Once an integrated circuit that includes a circuit such as circuit 100 is fabricated, the current 1101 through the constant current source becomes substantially fixed. Additionally, the voltage swings of the outputs are fixed. Generally, the voltage swings of the outputs will be substantially equal. In circuits such as circuit 100, the bias voltage applied to the base of the current source transistor Q3 is substantially fixed, although some sort of temperature compensation may be included. This bias voltage sets the amount of current through the current source, and the resulting voltage swings of the outputs, at fixed values.
The fixed magnitude of the voltage swings of the outputs in emitter coupled outputs, such as the outputs of the circuit 100 in FIG. 1, is typically set large enough to ensure satisfactory performance for a wide range of applications under worse case conditions, also taking into account imprecision in the magnitude of the voltage swings that may result due to limitations of the manufacturing process. Even though the magnitude of the output voltage swings is set large enough for most applications, some applications may require larger output current swings than is provided.
A significant shortcoming of prior art emitter coupled output circuits such as circuit 100 is that many applications do not require output voltage swings that are as large as the fixed output voltage swings provided, resulting in unnecessary power consumption and heat generation. For very fast and low jitter applications, the collector resistors that determine the output voltage swings must be set to match the impedance of the signal trace on the board, which will typically be 50 ohms. Thus, to achieve a 500 mV (single ended) swing, a minimum of 10 mA of current (500 mV divided by 50 ohms) would be required. Thus, the outputs of the circuit 100 may provide 500 mV voltage swings to an application requiring only 300 mV voltage swings, resulting in 4 mA of current being wasted for each of the outputs Y and YN. In an application with a 100 ohm line to line termination, the effective impedance is 25 ohms, which requires 20 mA of current (500 mV divided by 25 ohms) to achieve 500 mV voltage swings. In this case, if the outputs of the circuit 100 provide 500 mV swings to an application requiring only 300 mV voltage swings, 8 mA of current will be wasted for each of the outputs Y and YN. Conversely, another shortcoming of prior art emitter coupled output circuits such as circuit 100 is that some applications require output voltage swings that are larger than the fixed output voltage swings provided by these circuits.
Although prior art circuits, such as those disclosed in U.S. Pat. No. 3,760,200 of Taniguchi et al., provide for reduction of fluctuations of the amplitude of circuit outputs, these circuits do not have a default operating mode where connection of an external resistor or current source is not required, and they consequently require connection of an external resistor or current source to the integrated circuit containing the output circuitry at all times.